EnforcePermissionHelperDetector: @EP can only be used in Stub ancestors

Add helpers to identify the generated Stub class as well as methods that
inherit from it. Use those helpers where relevant, including
EnforcePermissionHelperDetector.

Bug: 261974131
Test: EnforcePermissionHelperDetectorTest

Change-Id: I68e572e6a8558be966450fcbf75eabbdb7ac6ae7